I have family near BWI, so I fly LAX-BWI about 1x/year.
If your employer will pay for F, then UA is the answer. DEN, SEA or LAX, plus the Air Canada network thru *A...
But my guess is that coach is in your future.
If you have (or want to have) status on United, E+ probably wins out vs WN. But since there are a lot of UA status pax in the greater DC area, you probably won't do to well on upgrades unless you get to PremierExecutive (1P) status, and even then they'll be rather costly. If you get to 1K status (100,000 miles/yr), the CR-1s will be very useful, especially on the westbound transcon.
WN is actually pretty comfortable in Y, but has the disadvantage of forcing you to get to the gate very early to stake out your preferred seat (even with an A BP). Plus, the transcon frequency is limited, and there are no redeye returns.